Onion skinning is a technique used by animators to see multiple frames at once. By using this technique, artists can easily see how to alter or adjust an image based on the previous image in the sequence. This technique is very useful for 3d animators to see previous animation frames or the later frames at once as well.
Slash is an add-on which aims to make using onion skinning technique easier for Blender users. It provides a clean and easy-to-use user interface and will make your animation skill to next level.